1. Field of the Invention
A light diffuser tip for controlling the light intensity distribution emanating from an optical fiber or fiber bundle.
2. Description of the Prior Art
An optical fiber generally consists of an optically transmissive core, a reflective cladding surrounding the core and an outer jacket which facilitates handling. There are several ways that the cylindrical diffusion of radiant energy from an optical fiber core can be accomplished. One way is to choose a ratio of the indices of refraction between the cladding and the core region of the optical fiber so that internal reflection within the core region is substantially less than total. This causes light to enter the cladding. If scattering centers are present in the cladding the light can radiate outward to emerge through the (preferably transparent) cladding.
Another way is to alter the interface between the optical fiber core and cladding to increase side radiation. Texturing the outer surface of the core region to provide a ground glass effect is one method commonly used. Another is positioning or embedding light scattering elements such as tiny particles at the surface-of the optical fiber core near the interface with the cladding. Light scattering particles can also be imbedded throughout the cladding to enhance the side delivery of radiation. Combinations of these measures are also possible.
Clark, in U.S. Pat. No. 4,336,809 describes a tissue photo irradiation system for use with hematoporphyrin dyes and derivatives thereof. Clark describes the use of an optical needle which serves as a linear radiator or a cylindrical diffuser and which can be coupled to an optical fiber by means of a conventional optical coupler. Clark's needle includes a fiber optic core that is generally internally reflecting. The core is surrounded by a cladding as generally known; but in an end region a different cladding surrounds the core to make it into a radiator instead of an internally reflecting transmitter. When the cladding contains scatterers, the "needle" or diffusion tip comprises a transparent core surrounded by a scattering layer in which the concentration of scatterers is homogeneous along its length.
McCaughan, Jr., in U.S. Pat. No. 4,660,925, incorporated herein by reference, describes a cylindrical diffuser tip that overcomes some of the problems with prior art diffuser tips. McCaughan Jr. suggests (column 4, lines 48-62) providing a tip surrounding the core of an optical fiber, the tip containing a gradient of scatterers which increases logarithmically in concentration along the fiber axis in a direction toward the polished tip of the optical fiber. To accomplish this, McCaughan, Jr. teaches a method for making such a tip comprising the steps of exposing (i.e. stripping the cladding away from) the core of an optical fiber near its tip, polishing the exposed core and repeatedly dipping the tip in a curable medium containing different concentrations of scatterers to allegedly increase the concentration of scatterers along the length of the exposed core. Such a method of repetitive coating followed by the step underlined above is inoperable to produce a longitudinal gradient of scatterers in a diffuser tip. This method produces a radial gradient in scatterer concentration; the concentration of scatterers varying radially with distance from the fiber core axis. Even if this method could, by further experimentation, be made operable, such a method would provide, at best, a discrete, step-wise concentration gradient which would only approximate a logarithmic gradient in the limit of infinite coatings.
The present optical fiber cylindrical diffuser tip technology is limited in clinical applications due to an output intensity distribution which may not uniformly illuminate a target surface which makes treatment dosimetry uncertain and clinical results inconsistent.
It is desirable, therefore, to provide a cylindrical diffuser for use as a termination on an optical fiber which overcomes most or all of the limitations stated above. In particular, it is desirable to provide a cylindrical diffuser tip for an optical fiber which provides a desired intensity distribution when immersed in a medium other than air, such as human tissue, and may be further customized to uniformly illuminate a target tissue presenting an irregular geometry.